1. Technical Field
The present invention relates to a method and apparatus for transmitting packets, the method and apparatus being capable of saving energy in a mobile ad-hoc network.
2. Related Art
With the ongoing development of wireless technology, a variety of mobile devices have been introduced. The mobile devices provide convenient portability but have a small display screen, an inconvenient interface, small battery capacity, etc. Batteries have been extensively studied. Since mobile devices are powered by a battery, efficient consumption of power is a critical consideration in the field of mobile devices. In particular, a Mobile Ad-hoc NETwork (MANET) saves energy in a power saving mode (PSM).
A mobile ad-hoc network is a network in which communication, such as data transmission and reception, is performed by routing between mobile terminals in a state where there is no network infrastructure. The ad-hoc network has no central control system, such as a base station or an access point. Mobile terminals directly transmit and receive data to and from each other. Accordingly, each mobile terminal participating in the network should act as a router, a server and the like to transmit and receive packets without the assistance of a base station or an access point.
Such an ad-hoc network is called an Independent Basic Service Set (IBSS). In the ad-hoc network, a mobile terminal is in direct communication with other mobile terminals present in a communication area. Generally, the ad-hoc network includes some mobile terminals that are active during a specific period with a specific intention. For example, the ad-hoc network may be temporarily set up in a conference room.
In the IEEE 802.11 wireless local area network (WLAN) standard, a distributed coordination function (DCF) is defined in a medium access control (MAC) protocol, which supports a mobile ad-hoc network. For saving power, the PSM is defined as a standard.
In the PSM, the mobile terminal has three power states: an off state, an awake state, and a doze state. In the off state, the mobile terminal does not operate, and thus consumes no power. In the awake state, the mobile terminal generally performs data transmission and reception while consuming necessary power. On the other hand, in the doze state, the mobile terminal does not perform data transmission and reception but remains in a standby state for a predetermined time period, during which less power is consumed than in the awake state.
In a DCF-based medium access control method, divided fixed periods of time are defined as beacon intervals. The fixed periods of time are used for data transmission. Each beacon interval includes an Ad-hoc Traffic Indication Message (ATIM) window and a data transmission window. The ATIM window is a period during which mobile terminals exchange setup information with each other to transmit and receive packets. The data transmission window is a period during which the mobile terminals actually transmit data packets based on information that is exchanged in the ATIM window.
In the ATIM window, each mobile terminal in the ad-hoc network is switched to the awake state and exchanges an ATIM frame with a neighboring mobile terminal so as to perform data transmission to the mobile terminal.
The mobile terminals, which transmit or receive the ATIM frame, stay in the awake state to transmit data in the data transmission window. On the other hand, mobile terminals that do not transmit or receive the ATIM frame are switched to the doze state to save power since they do not have to perform data communication. In the awake state, the mobile terminals occupy a medium through free competition to transmit and receive data packets. This data packet transmission and reception process continues until an ATIM window of the next beacon interval is initiated. Accordingly, one beacon interval is completed and data packet transmission is continued through a hop-by-hop process. The packet transmission and reception process of a mobile terminal in the above-described PSM method will be now described.
In the above-described packet transmission method, mobile terminals compete for data transmission through free competition in the data transmission window, and mobile terminals having fewer packets to transmit cannot be switched to the doze state because they cannot transmit packets stored in a buffer when continuously failing in transmission competition. This degrades energy saving efficiency.